Oil field operators demand access to a great quantity of information regarding the parameters and conditions encountered downhole. A wide variety of logging tools have been and are being developed to collect information relating to such parameters as position and orientation of the bottom hole assembly, environmental conditions in the borehole, and characteristics of the borehole itself as well as the formations being penetrated by the borehole. One of the instruments sometimes incorporated into logging tools is a magnetometer. Magnetometers are used to measure the strength and direction of magnetic fields, and are used in logging tools to measure the Earth's magnetic field to determine both the position of the tool as well as to identify magnetic anomalies in the surrounding strata. Such anomalies can be indicative of petrochemical deposits or other minerals of interest.
But as the sensitivity of magnetometers has increased, so has the design complexity of these instruments. Electronic devices, such as those used to control and monitor magnetometers, produce electromagnetic fields that can interfere with the magnetometer itself. One class of magnetometers developed to address this issue (for use in both oil field and non-oilfield environments) has been all-optical magnetometers, which do not incorporate electrical or electronic devices in the sensor itself, relying instead on changes in the optical properties of light transmitted through a gas-filled cell within the sensor. One example is a Frequency Modulated Bell-Bloom (FM BB) magnetometer that uses a sensor with an alkali vapor cell interrogated by an FM laser. With such magnetometers, the optical emitters, optical receivers, and their associated electronics can be distanced away from the sensor, thus reducing interference with the magnetic fields of interest. The optical emitters and receivers of this configuration are coupled to the sensor using optical fibers.
However, all-optical magnetometer sensors require that the polarization of the interrogating light be maintained and this can be very difficult to achieve the over extremely long fiber lengths used to couple surface electronics to downhole sensors, which in drilling/logging environments can extend to several kilometers. Defects and bends over such lengths, even in high quality polarity maintaining fiber, can significantly degrade the polarization of the laser light, and absent sufficient polarization the sensor will not operate properly.
It should be understood, however, that the specific embodiments given in the drawings and detailed description thereto do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed together with one or more of the given embodiments in the scope of the appended claims.